ABSORPTİON

Bringing the dirty effluent gas into contact with the scrubbing liquid and subsequently separating the cleaned gas from the contaminated liquid

Absorption is a basic chemical enginnering unit operation which in the APC field is reffered as scrubbing

They have wide use in controlling SO2, H2S, and light hydrocarbons

ABSORPTİON

_Wet scrubbers can be categorized into 3 groups:

1. Packed-bed counterflow scrubbers2. Cross-flow scrubbers 3. Bubble plate and tray scrubbers

SCRUBBER TYPES

Packed TowerSpray tower Venturi Absorber

CONCEPT OF ABSORPTİON

Gas absorption is the removal of one or more pollutants from a contaminated gas stream by allowing the gas to come into intimate contact with a liquid that enables the pollutatants to become dissolved by the liquid. The principal factor dictating performance is the solubility of the pollutants in the absorbing liquid.

The rate of transfer in the liquid is dictated by the diffusion processes occurring on each side of the gas liquid interface.

LİQUİD WASTE

Pollutants removed from the gas stream transferred into liquid phase whose disposal is another issue to deal with.

Therefore scrubber needs other units such as storage vessels, additives to treat the scrubbing liquid according to required discharge standards.

ABSORPTİON

Absorption units must provide large surface area of liquid-gas interface

Therefore the units are designed to provide large liquid surface area with a minimum of gas pressure drop

PACKED TOWER

PACKİNG MATERİAL AND SHAPES Packing material (must be inert) is designed to

increase the liquid-film surface Many geometric shapes are available : Raschig

ring, pall ring, berl saddle, tellerete etc.

PACKİNG MATERİAL AND SHAPES

PACKİNG MATERİAL PROPERTİES

ABSORPTİON THEORY Physically, the absorption of a pollutant gas

from a moving gas stream into an appropriate liquid stream is quite complex

Basically the transfer process into each fluid stream is accomplished by 2 mechanisms:

The pollutant species is transferred from the bulk of the gas stream toward the gas-liquid interface by turbulent eddy motions

Very close to the interface laminant flow is valid and transfer is accomplished by molecular diffusion

On the liquid side of the interface process is reversed

ABSORPTİON THEORY

ABSORPTİON THEORY On the basis of Fick's Law, the diffusion of one gas

(A) through a second stagnant gas B, NA, the molar rate of transfer of A per unit cross-sectional area is given by;

NA = -DAB (dcA/dz)/(1-(cA/c)

DAB: molecula diffusion coef. (m2/t)

cA: molar concentration of species A (mol/L)

c: molar concentration of the gas mixture (mol/L)

z: the direction of mass transfer (m)

DAB tables are available for a number of binary gas mixtures

ABSORPTİON THEORY Mass transfer rate per unit area for molecular

diffusion of A through a second liquid is given by: NA = -DL/z (cA2-cA1)

DL: liquid phase molecular diffusion coef. (m2/t)

cA2-cA1: concentration difference of A over the distance z

Typical values of DL for binary mixtures are tabulated in the literature.

THE EQUİLİBRİUM DİSTRİBUTİON CURVE

Before entering into details of mass transfer, let's summarize the method of presenting equilibrium data for a pollutant A distributed between liquid and gas phase

Inert Liquid Solvent

Inert carriergas

Inject solute A

P=c

Mole fraction in gas, yA

Mole fraction in lqiuid, xA

Exp. Equilib. Distribution

curve

THE EQUİLİBRİUM DİSTRİBUTİON CURVE

After sufficient time, no further change in the concentration of A in two phases. These concentration can be measured and converted into mole fraction xA in the liquid phase and yA in the gas phase

Inert Liquid Solvent

Inert carriergas

Inject solute A

P=c

Mole fraction in gas, yA

Mole fraction in lqiuid, xA

Exp. Equilib. Distribution

curve

MASS TRANSFER COEFFİCİENTS BASED ON INTERFACİAL CONCENTRATİONS

When mass transfer occurs in moving liquid and gaseous streams, it is difficult to evaluate the separate effects of molecular and turbulent diffusion

An alternative to this is to express NA for each phase in terms of mass transfer coefficient k and a driving force based on the bulk and interfacial concentrations for that phase

MASS TRANSFER COEFFİCİENTS BASED ON INTERFACİAL CONCENTRATİONS

For the liquid phase: NA = kL(cAi-cAL) = kx(xAi-xAL)

kL(is the liquid mass transfer coeff. Based on concentration, in length per unit of time, cAiis the concentration of A in the liquid phase at the interface, cALis the concentration of A in the bulk of the phase, in moles per unit volume.

kx is the liquid mass transfer coefficient based on mole fractions, in moles per units of time and length squared, xA is the mole fraction of A in the liquid interface, and xAL is the mole fraction of A in the bulk of the liquid phase

MASS TRANSFER COEFFİCİENTS BASED ON INTERFACİAL CONCENTRATİONS

For the gas phase: NA = kG(pAG-cAi) = ky(yAG-yAi)

kGis the gas phase mass transfer coeff. based on partial pressures, in moles/length2 time, pAGis the partial pressure of A in the bulk of gas phase pAi is the partial pressure of A in the gas interface

ky is the gas phase mass transfer coefficient based on mole fractions, in moles per units of time and length squared, yAG is the mole fraction of A in the bulk of the gas phase, and yAi is the mole fraction of A in the gas phase interface

MASS TRANSFER COEFFİCİENTS BASED ON INTERFACİAL CONCENTRATİONS

However this approach to determining NA is not practical since kx and ky are difficult to obtain and no way to measure the values of yAi and xAi experimentally since any attempt to do it will perturb the equilibrium between the two streams

OVERALL MASS TRANSFER COEFFİCİENTS When mass transfer rates are reasonably low, NA can

be expressed as: NA = KG(pAG-pA*) = Ky(yAG-yA*)

KG and Ky are local overall mass transfer coefficients

pA* : equilibrium partial pressure of solute A in a gas phase which is in contact with a liquid having the composition of cAL of the main body of the absorption liquid

yA*: defined similarly in terms of a liquid with mole fraction xAL of the bulk liquid

OVERALL MASS TRANSFER COEFFİCİENTS

slope=m'

Point P represents the state of the bulk phase of the 2 fluid streams, y

AG and x

AL.

The point M represents the state (y

Ai and x

Ai) associated with

equilibrium at the interface

The distance between P and C is a measure of the driving force.

OVERALL MASS TRANSFER COEFFİCİENTS

NA = KG(pAG-pA*) = Ky(yAG-yA*) This equation is usually restricted the resistance to mass transfer is primarily in the gas phase, which characterizes the majority of absorption problems in air pollution work

The solubility of the polutant gas normally determines the liquid that is chosen

The major physical problem is getting the pollutant to diffuse through the gas phase to the interface, consequently gas phase controls the process.

If the liquid phase controls: NA = KL(cA*-cAL) = Kx(xA*-xAL)

OVERALL MASS TRANSFER COEFFİCİENTS It is important to note that the quantities pA*,yA*,cA*,xA* do not represent any actual condition in the absorption process but are related in each case to a real concentration in one of the bulk fluids through the equilibrium data for the two-phase system. From the geometry of the previous figure:

yAG-yA*= yAG-yAi+(yAi-yA*)

yAi-yA*=m'(xAi-xAL)

yAG-yA*= yAG-yAi+m'(xAi-xAL)

1/Ky=1/ky+m'/kx

MASS BALANCES AND THE OPERATİNG LİNE FOR PACKED TOWERS

T = constP = constCross-sectional area, A

Gm,2

Gc

y2

Y2

Lm,2

Ls

x2

X2

Gm,1

Gc

y1

Y1

Lm,1

Ls

x1

X1

dz

Gm molar total gas flow

rate (carrier gas + pollutant)Gc molar inert carrier gas flow rateL

m molar total solvent

flow rate (solvent + absorbed pollutant)Ls molar solvent flow ratex is the liquid mole fraction of pollutant, y is the gas phase mole fraction of the pollutants, X is the liquid phase mole ratio and Y is the gas phase mole ratio

MASS BALANCES AND THE OPERATİNG LİNE FOR PACKED TOWERS

Mole fraction and mole ratio: X = x/(1-x) Y = y/(1-y) Subscript m denotes that rates are in the units

of mole basis The conservation of mass principle applied to

the pollutant species in terms of total mass flow rates at top and bottom yields:

Gm,1y1+ Lm,2x2 = Gm,2y2+ Lm,1x1

or Gm,1y1 -Gm,2y2 = Lm,1x1 -Lm,2x2

MASS BALANCES AND THE OPERATİNG LİNE FOR PACKED TOWERS

In Gm,1y1 -Gm,2y2 = Lm,1x1 -Lm,2x2 total gas and liquid flow rates are not equal at the top and the bottom of the column, therefore we cannot further simplify this equation.

When we write the equation in terms of the carrier gas and liquid solvent rates then:

GC,m(Y2-Y1) = LS,m(X2-X1) These two equations above gives a straight line

on Y-X coordinates with a slope of Lsm/GCm and called operating lines.

The operating line lies above the equilibrium line for absorption

For a stripping (removal of gas from liquid stream) the operating line must lie below the equilibrium line in order for the drving force to act from the liquid phase toward the gas phase

Dirty air

Dirty waterClean water

Clean air

THE MİNİMUM AND DESİGN LİQUİD- GAS RATİO

At the bottom and top of the absorber, parameters Gm,1, Gc, y1, Gm2, y2, and x2 are known.

We need to determine Ls, and x1 So we have one equation with 2 unknowns... However selection of one of these values,

obviously fixes the other. How to select a value?

THE MİNİMUM AND DESİGN LİQUİD- GAS RATİO

The minimum rate is highly undesirable. At this point driving force is almost 0. Hence it would take an infinetely tall absorber to accomplish the desired separationAs a general operating principle an absorber is typically designed to operate at liquid rates which are 30 to 70 % greater than minimum rate.

TOWER DİAMETER AND PRESSURE DROP PER UNİT TOWER HEİGHT

For a given packing and liquid flow rate in an absorption tower variation in the gas velocity has a significant effect on the pressure drop

As the gas velocity is increased, the liquid tends to be retarded in its downward flow, giving rise to term liquid holdup (LH)

A LH increases, the free cross-sectional area for gas flow decreases and pressure drop per unit height increases.

PROBLEMS WİTH HİGH GAS VELOCİTY Channeling: the gas or liquid flow is much greater

at some points than at others Loading: the liquid flow is reduced due to the

increased gas flow; liquid is held in the void space between packing

•Flooding: the liquid stops flowing altogether and collects in the top of the column due to very high gas flow

TO AVOID this condition experience dictates operating at gas velocities which are 40 to 70 % of those which causing flooding

FLOOD POİNT

The relationship between P/Z and other important tower variables-liquid and gas rates, liquid and gas stream densities and viscosities, and type of packing has been extensively studied on an experimental basis.

A widely accepted correlation among these parameters can be seen in below figure

Where G' and L': superficial gas and liquid mass flow rate defined as actual flow rates divided by the empty cross-sectional area of the tower.

L’/G’√(G/L-G)

L’: liquid mass flux (lb/s-ft2)

G’:gas mass flux (lb/s-ft2)

F:packing factor (ft2/ft3)

L:liquid viscosity, cp

gc: proportıonality constant, 32.17 ft-lb/s2-lbf

L:liquid density, lb/ft3

G:gas density, lb/ft3

GGLc

L

g

FG

)(

)'( 1.02

In Cooper and Alley’s book, Figure 13.6 can be used. Note that in Figure 13.6 Gx and Gy are liquid and gas flux (lg/s-ft2), respectively. In our notation G’ and L’ correspond to Gx and Gy

PACKİNG FACTOR F

The top line in the figure represents the general flooding condition for many packings. The flooding condition however has been found to vary as a function of the packing factor F (dimensionless packing factor tabulated below)

Recent studies showed that when F is in the range of 10 to 60, the pressure drop can be expressed by:

Pflood = 0.115F0.7

PACKİNG DATA

DETERMİNİNG TOWER DİAMETER

First abscissa value is calculated(L'/G')(pG/(pL-pG))0.5

Where this value intercepts the flooding line on Figure A, move horizontally to the left and read the value of the ordinate:

(G')2F(L)0.1/gc(pL-pG)pG

Calculate the G’ and take 30 to 70% of it to prevent flooding

Tower crossectional area: A = G/G‘

Evaluate the tower diameter

DETERMİNİNG EXPECTED PRESSURE DROP PER UNİT HEİGHT OF TOWER

First calculate actual G’ and L’ and then calculate the abscissa and the ordinate for use in Figure 13.6

From those values the intersection on the figure defines the pressure drop per foot of packed height

Another emprical correlation found in the

litrature for the P in packing when operating below the load point is

P/Z = 10-8m[10nL’/L](G’2/G) m and n are packing constants see Table 6.2

DETERMİNİNG TOWER DİAMETER AND EXPECTED PRESSURE DROP PER UNİT HEİGHT

OF TOWER

EXAMPLE

A packed tower is to be designed to remove 95% of the ammonia from a gaseous mixture of 8 percent ammonia and 92% air, by volume. The flow rate of the gas mixture entering the tower at 68 F and 1 atm is 80 lb-moles/hr. Water containing no ammonia is to be the solvent, and 1-in. Ceramic raschig rings will be used as the packing. The tower is to operated at 60% of theflood point and the liquid water rate is to be 30% greater than the minimum rate. Determine 1. The gas-phase flow rates, in lb-moles/hr, for the solute and carrier gas2. The mole ratios of the gas and liquid phases at inlet and outlet and the required water rate in lbmoles/hr.3.The gas and liquid rates (lb/hr) for carrier gas, solute gas, total gas, liquid solvent, solute in liquid, and total liquid4. The tower area and diameter5. The pressure drop based on the two methods given in the lecture notes.

EXAMPLERemoval efficiency: 95%

Effluent Stream Composition: 8% ammonia and 92% air

Gas T and P: 68F and 1 atm

Flowrate: 80 lb-moles/hr

Liquid phase: Containing no ammonia

EXAMPLE

X 0.0206 0.0310 0.0407 0.0502 0.0735 0.0962Y 0.0158 0.024 0.0329 0.0418 0.0660 0.0920

Determine composition of the liquid at the exit (X1)

(Inlet liquid concentration since pure water is used is x2=X2=0)

Use equilibrium data for ammonia-air-water mixtures which are given below for 68 F and 14,7 psia. :

In order to determine composition of liquid at the exit, we need to calculate the minimum solvent flow rate first.

By plotting X vrs Y at the equilibrium, we can evaluate the minimum solvent and then operating solvent rate.

In Cooper and Alley’s book, use Table B4 in the Appendix.

EXAMPLE

0

0,02

0,04

0,06

0,08

0,1

0 0,02 0,04 0,06 0,08 0,1

X, moles solute per mole solvent

y,

mo

les

so

lute

pe

r m

ole

c

arr

ier

ga

s

X2,Y2

(Lm,S/Gm,C)min= (0,087-0,00435)/0,092

Y1=0.087

Since the liquid rate is to be 30% greater than the miniumu rate

(Lm,S)/Gm,C)design = 1,30(0.90) = 1.17 mole/mole

Lm,S = Gm,C*1,17 = 1.17*73.6 = 86.1 lb moles/hr

0,90

0.092Y2=0.00435

EXAMPLE

Now, X1 can now be found.

1. Graphically by drawing operating line with a slope of 1.17 with starting point of (0, 0.00435) and the point crosses Y1=0.087 can be read. OR

2. From Lms/Gm,C = Y2-Y1/(X2-X1)=0.00435-0.087/(0-X1) = 1.17

X1 = 0.0707 lm mole A/lm mole water or x1 = 0.066 lb mole A /lb moles solution

0,90

0.00435

0,02

0,04

0,06

0,08

0,1

0 0,02 0,04 0,06 0,08 0,1

X, moles solute per mole solvent

y, m

ole

s so

lute

per

mo

le

carr

ier

gas

X2,Y2

(Lm,S/Gm,C)=1.17

Y1=0.087

X1: 0.0707

FLOW RATES

The gas and liquid rates: GC = 73.6*29 = 2134 lb/hrGA,1 = 6.4*17 = 109 lb/hrGA,2 = 0.32*(17) = 5.4 lb/hrLS = 86.1*18=1550 lb/hrLA,1 = GA= 109*0.95=104 lb/hr

Therefore: G1 = 2134 +109=2243 lb/hr bottomL1 = 1550 +104 = 1654 lb/hrG2 = 21345+5=2139 lb/hr topL2=1550 + 0 = 1550 lb/hr top

T = constP = constCross-sectional area, A

Gm,2

Gc

y2

Y2

Lm,2

Ls

x2

X2

Gm,1

Gc

y1

Y1

Lm,1

Ls

x1

X1

dz

TOWER AREATo determine the tower area, we need to use Figure flooding

correlation plot. Therefore we need to calculate gas and liquid phase densities

at the top and bottom of the tower. Since the ammonia content is very low in liquid phase, use the density of pure water, 62.3 lb/ft3 as the solution density through the tower.

For the gas phase assume ideal gas behavior: = P/RT = MwP/RTAt the top: Mw= yiMi = 0.00435*17 + 0.9957*29

= 28.95= 28.95*14.7/(10.73*528) = 0.075 lb/ft3

At the bottom Mw= 0.08*17 + 0.92*29 = 28.04= 28.04*14.7/(10.73*528) = 0.0728 lb/ft3

Now calculate the abscissa of Flooding Figure

TOWER AREA

TOWER AREA

PRESSURE DROPPressure drop can be determined from the flooding

figure or from an emprical equation

DETERMİNATİON OF AN ABSORPTİON TOWER HEİGHT

Height of a packed tower = f(the overall resistance to mass transfer between the gas and liquid phases, the average driving force and interfacial area)

Consider a differential height of the absorber dZ. In height dZ, the rate of mass transfer of species A

a: interfacial area available to mass transfer per unit volume of the packing

A: cross-sectional area of the tower

)'()()( yGAdyGdadZAN mA

TOWER HEİGHT

The equation can be also written for liquid resistance part.

)(

'

)(

'

)'()(

**

*

yyaPK

yG

yyaK

dyGdZ

yGddzyyaK

G

m

y

m

mAAGy

TOWER HEİGHT

To solve the above equation we can determine the overall value of Kya (Kga) based on experimental “pilot plant” operated with a certain packing and gas/liquid rate. The right side of the equation can be integrated from the knowledge of the operating line and equilibrium line chracteristics.

This method can be modified to deal with the “height of a transfer unit” and “the number of transfer units” by modifiying the equation somewhat

TOWER HEİGHT

To solve the above equation we can determine the overall value of Kya (Kga) based on experimental “pilot plant” operated with a certain packing and gas/liquid rate. The right side of the equation can be integrated from the knowledge of the operating line and equilibrium line chracteristics.

This method can be modified to deal with the “height of a transfer unit” and “the number of transfer units” by modifiying the equation somewhat

)(

'

)(

'** yyaPK

dyG

yyaK

dyGdZ

G

m

y

m

The equation can be expressed in terms of height of transfer unit (HTU ) and number of transfer units :

HTU is reaonably constant through the absorber and has unit of length. NTU is dimensionless.

55

1

*

' y

yOG

m

Z yy

dy

aPK

GZ

Tower Height

HTU or Hoy

NTU or Noy

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1

*

' y

yOG

m

Z yy

dy

aPK

GZ

x1, y1*

x1, y1

xZ, yZ*

xZ, yZ

For dilute gas streams, transfer unit equation can be simplified:

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Pure amineLm = 0.46 gmole/s

0.04% CO2

1.27% CO2

Gm = 2.31 gmole/sC* = 7.3% CO2 in amine

Q: A Packed tower using organic amine at 14 oC to absorb CO2. The entering gas contains 1.27% CO2 and is in equilibrium with a solution of amine containing 7.3% mole CO2. The gas leaves containing 0.04% CO2. The amine, flowing counter-currently, enters pure. Gas flow rate is 2.31 gmole/s and liquid flow rate is 0.46 gmole/s. The tower’s cross-sectional area is 0.84 m2. KOGa = 9.34×10-6 s-1atm-1cm-3. The pressure is 1 atm. Determine the tower height that can achieve this goal.

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Absorption of concentrated vapor

Mole balance on the controlled volume

)'()'(0 xLdz

dyG

dz

dmm

Gas flux

y

GG mm 1

1'' 0

Liquid flux

xLL mm 1

1'' 0

1

1

0

0

1

1

1

1

0

0

1

1

11'

'

11

11'

'

1

xx

xx

G

L

yy

xx

xx

G

L

yy

y

m

m

m

m

x1, y1

x1, y1*

xZ, yZ*

xZ, yZ

EXAMPLE A 1‐ft diameter packed column is used to scrub a

soluble gas (MW = 22) from an air‐gas mixture. Pure water enters the top of the column at 1000 lbm/hr. The entering gas stream contains 5% soluble gas and 95% air. Ninety‐five percent of the soluble gas is removed. Both the operating line and equilibrium curve may be assumed to be straight. The equation for the equilibrium curve is y = 1.2x, where x, y = mole fractions. The entering gas mixture flow rate is 800 lbm/hr. The column operates at 30 °C and 1 atm, and

Kya = 4.29 lbmol/hr‐ft3‐Δy φ

EXAMPLE

Calculate or find:

a) Concentration of the soluble gas in the effluent liquid if the column is operated at

minimum liquid flow rate

b) Concentration of soluble gas in the liquid at a point in column where y = 0.02

c) Height of packed section, ZT

d) Hoy

e) Whether columnis in danger of flooding if it is packed with 1⁄2‐in. ceramic Raschig

rings

Solution

Solution

Solution

Solution

Solution

Solution

Solution

Solution

Solution